MicroRNAs (miRNAs) are small (19-25 nucleotides) untranslated RNAs that serve as regulators of messenger RNA expression. For the most part, miRNAs bind to complementary regions in the 3' untranslated regions (UTRs) of target mRNA, and either cause mRNA degradation or prevent translation. MiRNAs have been found to be expressed in cell-specific manner, at specific developmental stages, as well as differentially expressed in disease states. MiRNA profiling in the post-natal mouse mammary gland has revealed alterations in miRNA expression at different key developmental stages (virgin, pregnancy, lactation and involution), strongly suggesting that miRNA play a regulatory role in mammary gland development. However, mechanisms by which miRNAs are regulated in the mammary gland and their specific functional roles at critical stages of development remain unexplored. Therefore, the overall goal of this project is to determine the hormonal regulation and functional roles of miRNAs during secretory activation in the mammary gland. Because of the dramatic metabolic changes occurring between pregnancy and lactation, the mammary gland is a unique model to study the Impact of miRNA on metabolism genes. Our specific aims will test the hypothesis that miRNAs represent an important mechanism regulating secretory activation by influencing translation of genes encoding glucose transporters, lipid synthetic enzymes and milk proteins. We have observed that specific miRNAs that decrease between mid-pregnancy and lactation, target genes encoding enzymes involved in fatty acid synthesis, transporters for glucose uptake and a translation initiation factor, elF4e. A decline in these miRNAs relieves repression and increases translation of target genes. Additionally, since during pregnancy, progesterone and its receptors inhibit terminal differentiation and secretory activation until parturition has occurred, we will determine whether ligand activated PR acts through miRNAs to mediate this inhibition. We will explore whether progesterone receptors regulate expression of miRNAs and, conversely, whether miRNAs mediate down regulation of progesterone receptors. We will manipulate miRNAs in vitro via lentiviral transduction into primary MECs prior to 3D culture, in vivo using adenovirus injected intraductally into the mammary glands of mice, and by mammary gland-specific deletion of miRNA of interest using Cre recombinase. These studies will provide direct evidence for the role of miRNAs in modulating metabolism in the mammary gland. RELEVANCE (See instructions): In the mammary gland, lactation requires a profound and rapid developmental switch termed secretory activation. In our study we investigate the hypothesis that microRNAs (small non-coding RNAs that simultaneously control translation of many coding RNAs) are master regulators of the dramatic metabolic changes that occur between pregnancy and lactation. The goal of this project is to define the hormonal regulation and functional roles of microRNAs during secretorv activation in the mammary gland.